The IS recording was compared to other audio from Abu Rumaysah by one of the UK's top voice analysts and showed a "complex pattern of similarities" between the two.

Dr Frederika Holmes, who has more than 25 years' experience as an independent consultant in forensic and phonetic analysis of speech and language samples, said: "There was considerable overlap in accent features across the two samples, and both samples could be plausibly within the range of a single speaker."

Image Caption:Forensic speech analyst Dr Frederika Holmes

Abu Rumaysah, also known as Siddartha Dhar until his conversion to Islam a decade ago, skipped bail and fled to Syria with his wife and children in 2014.

He has emerged as a prime suspect for the man behind the mask in the IS video.

Video:How Are Jihadis Unmasked?

In the 10 minute film, one of five masked militants - speaking with an obvious British accent - criticises UK government policy and Prime Minister David Cameron, before the men shoot dead five hostages on camera.

Dr Holmes said: "The rehearsed and dramatic nature of this sample means that the recorded speech is likely to differ from the speaker's habitual speech patterns across a range of parameters. These factors have been taken into account in carrying out the analysis and formulating the conclusions."

The analysis of the YouTube recording showed a pattern of speech which was "much more informal and natural in style than the questioned recording (ISIS video). The recording contained ample quantity and range of speech for the purposes of analysis."

Dr Holmes said: "Comparison of the speech of the speaker in the two recordings revealed a complex pattern of similarities.

"Overall the accent of both speakers could be characterised as Multicultural Urban British English.

Video:Inside IS Weapons Lab

"There was considerable overlap in accent features across the two samples, with both London/Estuary and Asian British variants occurring in both samples. Both samples could be plausibly within the range of a single speaker."

The acoustic pitch of the two samples also displayed a close correspondence and is "consistent with both samples originating from a single speaker".

Dr Holmes said her overall conclusion was that "both phonetically and acoustically, the speech of the questioned male speaker (ISIS Video) is 'consistent' with the speech in the reference recording (Abu Rumaysah YouTube video)".

She said: "The distinctiveness of the features found to match across the two samples was judged to be 'distinctive'. This finding 'gives support' to the hypothesis that the speaker in both the recordings is the same speaker."

Shadow Home Secretary Andy Burnham has demanded a detailed investigation into the apparent breakdown in procedures which allowed Abu Rumaysah to flee the UK.

Video:Where Did Islamic State Come From?

The terror suspect and former bouncy castle salesman had been arrested in September 2014, in an operation against the now banned militant group Al Muhajiroun.

He was granted bail on the condition he hand in his travel documents, but disappeared the next day, before turning up in Syria a few weeks later.